Plaster

While softer and more liable to be damaged, plaster of paris is ideally suited to indoor life.

It breathes better than concrete, is lighter in weight and is easier to hang on fragile indoor surfaces such as paneling or drywall.

Plaster also shows up detail... amazing details. It has been claimed by some that cellular patterns can be seen in the skin impressions!

Concrete

Concrete is much more durble, and can easily be set in the out-of-doors. While not invincible, concrete is far more able to take the bumps and knocks of being displayed than plaster. Concrete pieces should be hung using heavy-duty hardware.

Concrete is heavier than plaster, and costs more to ship. In addition, the casting of a plaster piece ties up the mold for a day or more, and thus costs a little more to purchase. Plaster pieces can be cast in quick succession.

Concrete also shows great detail, but is very likely to show a few blemishes as well such as a fall out (where the concrete does not form) or air small bubbles.